CCNE1 and breast neoplasm: While not an all-inclusive list, studies using established breast tumor cell line and patient-derived xenograft models as well as retrospective analysis of clinical trial data have shown that resistance to CDK4/6 inhibition is associated with downregulation of RB expression, activation of interferon (IFN) signaling, FGFR1 amplification, overexpression of CCNE1 and overexpression/activation of CDK2/CYCLIN E1 complexes [187–189, reviewed in 190, 191].